This invention relates to a process for producing a polyamide-imide resin which can give a varnish having a high resin content, excellent in storage stability and is useful particularly as a varnish for heat-resistant electric wire.
In the prior art, as polyamide-imide resins used in varnishes for heat-resistant electric wire, there have heretofore been used those which are obtained by using N-methylpyrrolidone (NMP) as a solvent for synthesis, have a reduced viscosity (concentration: 0.5 g/dl, solvent: dimethylformamide, measurement temperature: 30.degree. C.) of more than 0.4 and have a sufficiently increased molecular weight. On the other hand, since the solution viscosities of varnishes for electric wire are set at about 30 poises (30.degree. C.) in the case of die coating because of restriction as to coating workability, the resin content of the above-mentioned polyamide-imide resins with a high molecular weight satisfying this requirement has an upper limit of about 30% by weight even if a good solvent NMP is used. Therefore, when such polyamide-imide resins with a high molecular weight are used in varnishes for electric wire, a large amount of expensive NMP must be used, and this poses a problem from the viewpoint of the cost.
One method for reducing the cost by decreasing the used amount of NMP and increasing the resin content is to lower the molecular weight of the resin. However, when the molecular weight of a polyamide-imide resin obtained from a diisocyanate and a tricarboxylic acid anhydride is lowered so that the reduced viscosity of the resin may be 0.4 or lower, the terminal functional group concentration of the resin increases, so that the viscosity of the resulting varnish increases gradually with the lapse of time, which results in causing a problem of marked lowering of the storage stability. In the case where the viscosity has increased as days go by, when the resin is used, for example, as a varnish for electric wire, there are caused inconveniences such as the alteration of initially set coating conditions and the adjustment of the viscosity by diluting the varnish having an increased viscosity with a solvent, and the characteristics of a protective coating film formed by volatilizing the solvent sometimes vary.
There is also a proposal aiming at removing these disadvantages, on a process for producing a stabilized polyamide-imide resin capable of having a high resin content in which terminal functional groups are masked with a specific active-hydrogen-containing compound. This process is greatly improved in the storage stability of a polyamide-imide resin having a lowered molecular weight, but is required to employ more strict stabilizing technique for polyamide-imide resins which have a lowered molecular weight for making the reduced viscosity 0.3 or lower and have a greatly increased resin content. That is to say, it is necessary to devise a stabilizing method so that polyamide-imide resins stabilized in such a low molecular weight region may show a sufficient cure reactivity at the time of baking and curing. Particularly when there is used such an active-hydrogen-containing compound wherein the terminal functional groups are masked by thermally irreversible bonding groups in a usual baking temperature range, the resulting resin is greatly lowered in cure reactivity, though it is excellent in storage stability.